This invention relates generally to production of ethylene, and more particularly, to high yield process and apparatus for conversion of ethane to ethylene.
Ethylene is one of the most important building block petrochemicals produced in the United States and the world. It is usually produced by thermally cracking hydrocarbon feedstocks ranging from ethane to heavy gas oils. There are other production techniques under investigation for producing ethylene using catalytic systems, auto-catalytic with chlorine and oxygen, and various combinations of such processes.
The most economically viable process currently in industrial use is the pyrolysis of ethane, or ethane/propane mixtures, or light hydrocarbon liquids in the boiling range of 100.degree.-150.degree. F. This process is usually carried out in a high temperature pyrolysis furnace where steam and hydrocarbons are preheated to 300.degree. to 400.degree. C., and then passed through a cracking coil made from very high alloy CrNi steel, where the combined stream is heated to 700.degree.-900.degree. C. The typical yield obtained from such a process treating ethane is as shown in Table 1. The overall conversion of ethane is limited to 55-60 weight percent due to equilibrium and coking considerations and temperature limitations of the metal reactor tube. The unconverted ethane is typically recovered and recycled to permit an overall ultimate conversion of ethane to ethylene in the range of 70-82 weight percent. The by-products produced, as shown in Table 1 below, are hydrogen, methane, propylene, and heavier compounds, all of which require separation from the ethylene product which is usually required to be 99+% pure for use in a commercial process.
TABLE 1 ______________________________________ COMPARISON OF PRODUCT YIELDS IN ETHANE CRACKING PILOT VERSUS INDUSTRIAL UNIT wt % Pilot Plant Industrial ______________________________________ H.sub.2 3.71 3.71 CO/CO.sub.2 0.26 CH.sub.4 2.99 3.35 C.sub.2 H.sub.2 0.20 C.sub.2 H.sub.4 48.7 48.68 C.sub.2 H.sub.6 39.0 39.27 C.sub.3 H.sub.6 1.05 1.07 C.sub.3 H.sub.8 0.21 C.sub.4 H.sub.6 0.99 1.12 C.sub.4 H.sub.8 0.21 C.sub.4 H.sub.10 0.3 C.sub.5 1.85 1.6 Pressure atm. abs. 1.55 1.9 Steam dilution kg/kg 0.4 0.4 Outlet temp C. 840. 835. Conversion % 59.1 59.87 ______________________________________
There is need for an improved process providing significantly higher, or enhanced, ethylene yields.